Sirius Binary Stars
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Σείριος''Seirios'' ("glowing" or "scorcher"). The star has theBayer designation Alpha Canis Majoris (α CMa). What the naked eye perceives as a single star is actually a binary star system, consisting of a white main sequence star of spectral type A1V, termed Sirius A, and a faint white dwarf companion of spectral type DA2, called Sirius B. The distance separating Sirius A from its companion varies between 8.1 and 31.5 AU.[17] Sirius appears bright because of both its intrinsic luminosity and its proximity to Earth. At a distance of 2.6 parsecs (8.6 ly), as determined by the Hipparcos astrometry satellite,[18][5][19] the Sirius system is one of Earth's near neighbors; for Northern-hemisphere observers between 30 degrees and 73 degrees of latitude (including almost all of Europe and North America), it is the closest star (after the Sun) that can be seen with a naked eye. Sirius is gradually moving closer to the Solar System, so it will slightly increase in brightness over the next 60,000 years. After that time its distance will begin to recede, but it will continue to be the brightest star in the Earth's sky for the next 210,000 years.[20] Sirius A is about twice as massive as the Sun and has an absolute visual magnitude of 1.42. It is 25 times more luminous than the Sun[7] but has a significantly lower luminosity than other bright stars such as Canopus or Rigel. The system is between 200 and300 million years old.[7] It was originally composed of two bright bluish stars. The more massive of these, Sirius B, consumed its resources and became a red giant before shedding its outer layers and collapsing into its current state as a white dwarf around120 million years ago.[7] Sirius is also known colloquially as the "Dog Star", reflecting its prominence in its constellation, Canis Major (Greater Dog).[12] Theheliacal rising of Sirius marked the flooding of the Nile in Ancient Egypt and the "dog days" of summer for the ancient Greeks, while to the Polynesians it marked winter and was an important star for navigation around the PacSirius A has a mass double that of the Sun.[7][75] The radius of this star has been measured by an astronomical interferometer, giving an estimated angular diameter of 5.936±0.016 mas. The projected rotational velocity is a relatively low 16 km/s,[10] which does not produce any significant flattening of its disk.[76] This is at marked variance with the similar-sized Vega, which rotates at a much faster 274 km/s and bulges prominently around its equator.[77] A weak magnetic field has been detected on the surface of Sirius A.[78] Stellar models suggest that the star formed during the collapsing of amolecular cloud, and that after 10 million years, its internal energy generation was derived entirely from nuclear reactions. The core became convective and utilized the CNO cycle for energy generation.[76] It is predicted that Sirius A will have completely exhausted the store of hydrogen at its core within a billion (109) years of its formation. At this point it will pass through a red giant stage, then settle down to become a white dwarf. Sirius A is classed as an Am star because the spectrum shows deep metallic absorption lines,[79] indicating an enhancement in elements heavier than helium, such as iron.[70][76] When compared to the Sun, the proportion of iron in the atmosphere of Sirius A relative to hydrogen is given by ,[9] which is equivalent to 100.5, meaning it has 316% of the proportion of iron in the Sun's atmosphere. The high surface content of metallic elements is unlikely to be true of the entire star, rather the iron-peak and heavy metals are radiatively levitated towards the surface.[76] With a mass nearly equal to the Sun's, Sirius B is one of the more massivewhite dwarfs known (0.98 solar masses[80]); it is almost double the 0.5–0.6solar-mass average. Yet that same mass is packed into a volume roughly equal to the Earth's.[80] The current surface temperature is 25,200 K.[7]However, since there is no internal heat source, Sirius B will steadily cool as the remaining heat is radiated into space over a period of more than two billion years.[81] A white dwarf forms only after the star has evolved from the main sequence and then passed through a red giant stage. This occurred when Sirius B was less than half its current age, around 120 million years ago. The original star had an estimated 5 solar masses[7] and was a B-type star (roughly B4-5)[82][83] when it still was on the main sequence. While it passed through the red giant stage, Sirius B may have enriched the metallicity of its companion. This star is primarily composed of a carbon-oxygen mixture that was generated by helium fusion in the progenitor star.[7]This is overlaid by an envelope of lighter elements, with the materials segregated by mass because of the high surface gravity.[84] Hence the outer atmosphere of Sirius B is now almost pure hydrogen—the element with the lowest mass—and no other elements are seen in this star's spectrum.[85]